1. *tlhIngan Hol jatlh chaH* cannot be an instance of the "prefix trick" (meaning "They speak Klingon to them") because there is another possible interpretation, i.e. a direct quotation with no object, so no "prefix trick" is being used and the sentence can only mean "They say: 'Klingon language'". Right?
Right.
2. However, I don't understand why *Holmey law' lujatlh* couldn't be an instance of the "prefix trick" (I'm referring to De'vID's answer here: http://lists.kli.org/pipermail/tlhingan-hol-kli.org/2022-June/063074.html). There is no other interpretation of the verb prefix which makes sense. It is an usual error to forget *lu-* in informal speech, but not to use it when it isn't required, so its "unexpected" presence would point out to the *prefix trick* in the same way the presence of *qa-* point to it in *paq qanob*. Or am I missing something?
Understand that no one in that thread was speaking in terms of absolute rules. I was talking about ambiguity and uncertain interpretations. De'vID was a little more certain that Holmey law' lujatlh would just be considered an error.
3. We know now that the indirect object can also be explicit when using the "prefix trick" (*loDnI'Daj vavDaj je ja' qeylIS*). Is it possible to have both an explicit direct *and* indirect object with the "prefix trick"?
No.
How would it look like? Probably not so: *loDnI'Daj vavDaj je lut ja' qeylIS*, since *loDnI'Daj vavDaj je lut* could be interpreted as a noun-noun construction, ruling out the *prefix trick*. But what about *SoH paq qanob*? (*SoH paq* cannot be a noun-noun construction, right?).
The prefix trick is only used when the indirect object is not said explicitly. It's either SoHvaD paq vInob or paq qanob.
I wouldn't jump straight to the conclusion that SoH paq
cannot be a noun-noun construction. When dealing with directions
and areas, we get constructions like jIH 'em area
behind me. I believe we say things like jIH 'em
instead of 'emwIj because I do not actually possess 'em,
so possessive suffixes are inappropriate. So while phrases like SoH
paq you book aren't going to be normal, they would
still be understood in the same way: it's not paqlIj your
book; a SoH paq is an extraordinary way to refer to
a book about you. It's not good grammar, but it's not downright
unintelligible, either.
4. *toDuj lutraj quv lutraj je / QoymeH tlhIngan tuqmey / tIja'* (*paq'raD* 6, 4-6): Actually, *tI-* could here refer to *toDuj lutraj quv lutraj je*, making it impossible to interpret it as an instance of the "prefix trick", but in 7-9 (*DaH peHarghchoH / DaH molor yISuvchoH / tIja'*) it is clear that *tI-* refers to *tlhIngan tuqmey* ("prefix trick" in a direct quotation), so the context suggests that the first *tI-* is also an instance of the "prefix trick", doesn't it? And if so, is it right to use *ja'* only with an indirect object when it isn't in a direct quotation? ("Tell them so that the Klingon tribes hear your story of courage and your story of honor").
I don't think you're interpreting those lines correctly.
Tell them, "Start a major battle now! Start Fighting Molor now!"
This is an instance of quotation. The quotation is not any object of any kind; it is just another sentence pushed against the sentence that refers to speaking. The prefix trick may or may not be in use here (its use in ja' quotations has never been fully solved, in my opinion), but this stanza follows the pattern of ja' completely.
-- SuStel http://trimboli.name